1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer provided with an exposure device.
2. Description of the Prior Art
Generally, a laser printer transfers a toner image, in an image forming and transferring portion, to a recording paper sent from a paper supply device including a paper supply roller and the like by paper transport means including a transport roller and the like and, then, allows the tonor image to be melted and fixed on the recording paper in a fixing device. The image forming and transferring portion is made up of a process unit including a photosensitive member, a charging device, a developing device, a cleaning device, and the like, an exposure device for radiating light in accordance with printing data on the photosensitive member which has been charged to thereby form an electrostatic latent image thereon, and a transfer device for transferring a visible image (tonor image), which has been developed from the electrostatic latent image on the photosensitive member by the developing device, onto the recording paper.
A frame of the image forming apparatus, in general, is made up of a body frame in which various parts other than the exposure device are to be fixed being positioned in their places, an optical frame in which the exposure device is to be fixed being positioned in its place, and an outer case for covering over the parts. Each frame will be described below.
Component parts such as the paper supply device (a paper supply roller and the like), paper transport means (a transport roller and the like), the process unit, the transfer device, and the fixing device are each fixed in the body frame which has a high rigidity by means of their respective positioning references. Usually, the body frame is formed of plate iron or synthetic resin such as acrylonitrile-butadiene-styrene resin (hereinafter referred to as ABS resin).
The exposure device is mounted on the body frame such that it is located above the image forming and transferring portion by means of positioning members (positioning references). Optical parts constituting the exposure device i.e., a polygon mirror, an F.theta. lens, a reflecting mirror, and the like are extremely vulnerable to dirt because the quality of an image is apt to be easily deteriorated by dirt. Therefore, they are housed in a box-formed optical frame so that tonor dusts and the like are prevented from attaching to them. Since the optical parts are vulnerable to heat and their fixed positions with respect to each other are required to be highly precise, the optical frame is generally formed of a synthetic resin exhibiting least deformation when subjected to heat and capable of being shaped very accurately, such as polycarbonate.
Leaving each of the component parts fixed on the body frame bare of a casing is problematic in view of their external appearance, upkeeping of the functions of the component parts, safety in operation of the apparatus, and so on. Therefore, the body frame and the component parts fixed to the frame member are covered with an outer case. The outer case, in view of its function, is not required to be so rigid as the body frame or the optical frame and, hence, different materials from those of the body frame and the optical frame, such as ABS resin, are used to form it chiefly from the point of view of the good appearance it provides.
The outer case has an opening formed at its top so that attaching and detaching of the process unit, disposal of jammed paper, and the like may be facilitated and the case has a movable cover to open and shut the opening at the top. Sometimes, the exposure device is installed on the movable cover for such reasons as ease of attaching and detaching the process unit.
There are some problems with such a conventional image forming apparatus. First, it is required of modern image forming apparatus to be smaller in size and lower in cost as much as possible so that they can be used in an ever expanding field of utilization. Therefore, it is attempted to decrease the number of parts and reduce the space of installation of the photosensitive member and the like. However, there are limits in succeeding in such attempts and it is the present situation that the demands for downsizing and cost reduction are not fully satisfied.
Second, although improvements in the quality of images are also demanded of the image forming apparatus, it is difficult to meet such demands. More specifically, since the optical parts of the exposure device are fixed on the body frame by means of the optical frame, positioning members, and the like, it is difficult to achieve precise positioning of the parts with respective to the photosensitive member because the positioning is affected by such factors as dimensional accuracy of the positioning members and the like and thermal deformation. Especially, when the exposure device is installed on the movable cover, such factors as thermal deformation of the movable cover and the play in the rotation supporting structure are added to the above described factors and, hence, it becomes extremely difficult to achieve high positional accuracy of the exposure device with respect to the photosensitive member. To overcome such difficulties, it is considered effective to form the positioning members and the like of a material with very high rigidity or fix them robustly to the body frame. However, this leads to increase in cost and also makes it difficult, when, for example, attaching and detaching the process unit, to shift the exposure device such that it does not interfere with the movement of the process unit.